FIG. 11 shows a related art example of a constant voltage circuit using a series regulator. The constant voltage circuit shown in FIG. 11 includes a reference voltage generating circuit 101 for generating a predetermined reference voltage Vr and outputting the generated voltage Vr, an output transistor M101, an error amplifier circuit 102 including MOS transistors M102-M106, and resistors R101, R102 for detecting output voltage (hereinafter referred to as “output voltage detection resistors”). The error amplifier circuit 102 amplifies the voltage difference between the divided voltage Vfb divided by the output voltage resistors R101, R102 and the reference voltage Vr output by the reference voltage generating circuit 101, outputs the amplified voltage to a gate of the output transistor M101, and controls the output transistor M101 so that the output voltage Vo is stabilized at a predetermined voltage.
In recent years and continuing, it is desired to reduce the voltage difference (input/output voltage difference) between input voltage Vdd and the output voltage Vo as much as possible for reducing power consumption at the output transistor M101, to thereby reduce the power consumption of a device. It is also desired that the current flowing in the output voltage detection resistors R101, R102 be reduced as much as possible for reducing the consumption current inside the IC (Integrated Circuit). In order to reduce the difference between input voltage and output voltage, a transistor having a high driving capability is to be used for the output transistor M101. Furthermore, the threshold voltage of the output transistor M101 is reduced by shortening the length L of the gate of the output transistor M101 and increasing the width W of the gate of the output transistor M101.
In one exemplary related art case, there is a constant voltage circuit that can stabilize output voltage even where the current flow is low or null when operating with a low supply voltage (See for example Japanese Registered Patent No. 3643043). FIG. 12 is a circuit diagram showing such a constant voltage circuit. By adding a pseudo load circuit that supplies a predetermined current from the output transistor M111 in the circuit shown in FIG. 12, output voltage VOUT can be prevented from increasing even when no current flows in the load RL.
Here, however, a leakage current may occur in an off-state in a case of using a finely fabricated MOS transistor having a short gate length L or an MOS transistor having a small threshold voltage. Furthermore, a current leak of several μA may occur in a case of using a large MOS transistor having large gate width W and gate length L even where voltage Vgs between the gate and source. In a case where current flows to a connected load as in the circuit shown in FIG. 11, such leaking current has no effect on the output voltage since the leaking current can flow to the load. However, in a state where current flowing to the load ranges from 0 μA to several μA (i.e. almost no load), the leaking current, being unable to flow outside, flows to the output voltage detection resistors R101 and R102. Although it is possible to ignore the leaking current in a case where the leaking current is less than the current that steadily flows to the resistors R101 and R102, a large leaking current causes an increase of the output voltage Vo. Thus, the current flowing to the output voltage detection resistors R101, R102 cannot be reduced to an amount no greater than the leaking current of the output transistor M101, and reduction of power consumption cannot be accomplished.
FIG. 13 shows an example of temperature characteristics of a current i101 output from the output transistor M101 in a case where the constant voltage circuit shown in FIG. 11 is in a no load state. In the example shown in FIG. 13, the input voltage Vdd is 5V, the output voltage is 1V, and the current flowing to the output voltage detection resistors R101 and R102 is approximately 0.2 μA.
Although FIG. 13 shows a relatively steady current flowing in the range between low temperature and normal temperature, the above-described current leak occurs in the high temperature area.
FIG. 14 shows temperature characteristics of the output voltage Vo and the gate voltage of the output transistor M101 in a case where the constant voltage circuit of FIG. 11 is in a no load state.
As shown in FIG. 14, all the leaking current of the output transistor M101 flows into the output voltage detection resistors R101 and R102 since the current flowing to the load is 0 μA. Although the output transistor M101 attempts to regulate the current by switching to an off state (disconnected state), the gate voltage V101 of the output transistor M101 becomes substantially equal to the input voltage Vdd (5V) around 75° C. The output transistor M101 cannot control the output voltage Vo in a high temperature area of no less than 75° C. such that the output voltage Vo increases in proportion to the leaking current of the output transistor M101.
Although it is possible to increase the length L of the gate of the output transistor M101 or increase the threshold voltage of the transistor M101 for controlling the leaking current, such methods causes the difference between input voltage and output voltage to increase and result in large power consumption by the output transistor M101. Furthermore, with the configuration shown in FIG. 12, there is a problem where consumption current during a steady state increases due to the constantly operating pseudo load circuit 111.